1. Field of the Invention
The present invention relates to a charged particle beam apparatus that irradiates a charged particle beam on a sample to be observed and performs an observation or a measurement of the shape, material or the like of the sample using physical phenomena such as secondary electron generation or the like resulting from the irradiation, and more particularly, to a charged particle beam apparatus that synthesizes signals obtained by a plurality of scans to form a synthesized signal.
2. Background Art
When an electron beam is accelerated and irradiated on a sample in a vacuum, depending on the surface condition of the sample, secondary electrons (SE) or backscattered electrons (BSE) are emitted from the sample surface. A scanning electron microscope (SEM) uses a phenomenon wherein an emission of secondary electrons or the like varies depending strongly on the shape or the like of the sample, and performs imaging of the surface shape of the sample. The electron beam irradiated at this point is focused using an electron lens so that a focus position coincides with the sample surface.
A configuration such as described above enables irregularities on the sample to be captured as a clear image. With such a scanning electron microscope, the generation efficiency of secondary electrons is very poor and there is a possibility that the scanning electron microscope will be strongly affected by extremely irregular noise such as white noise. Therefore, there are cases where a single scan within an imaging area is unable to produce sufficient signal amount and only images with extremely poor S/N ratios are obtained.
Patent Document 1 and Patent Document 2 describe techniques for suppressing the influence of noise such as described above by scanning the same imaging area a plurality of times and averaging signals obtained as scanning results. Performing averaging enables irregularly generated noise to be suppressed to a certain degree. In addition, Patent Document 2 describes a technique for controlling noise reduction effects by appropriately adjusting the signal intensity of images to be accumulated.
In the description of the present specification, image signals corresponding to an entire imaging area obtained in each single scan may occasionally be referred to as one frame, an average image signal over two scans as a two-frame image, an average image signal over four scans as a four-frame image, and so on.
[Patent Document 1] JP Patent Publication (Kokai) No. 9-330679 A (1997) (corresponding to U.S. Pat. No. 5,929,439)
[Patent Document 2] JP Patent Publication (Kokai) No. 2000-182556 A